1. Field of the Invention
The present invention generally relates to gas turbine engines, and more particularly, to the relief of thermal stresses in an aerodynamic surface of a gas turbine engine. The present invention is particularly suited for relieving thermal stress in a fuel nozzle of a gas turbine engine combustor.
2. Description of the Prior Art
It is well known to use aerated fuel nozzles for atomizing fuel in a combustion chamber of a gas turbine engine. Such nozzles generally comprise a tubular cylindrical head or outer air swirler defining an array of circumferentially spaced-apart air passages to pass pressurized compressor discharged air at elevated temperatures into the combustion chamber of the engine to atomize the fuel film exiting from the tip of the spray nozzle.
It has been found that such fuel nozzles suffer from low cycle fatigue cracking at the thinnest portion of the webs between the air passages of the nozzle head. This cracking is caused by a thermal gradient existing from the surfaces of the nozzle, which are in contact with the hot pressurized air, to the nozzle core surfaces, which are cooled by the fuel, the temperature of which is less than 200xc2x0 F. as compared to temperatures as high as 1000xc2x0 F. for the hot pressurized air flowing through the air passages.
One approach to relieve the stresses in the nozzle head has been to separate the head or outer swirler into two radial components to separate hot from cold material. However, this solution is relatively expensive and increases the number of the pieces composing the spray nozzle tip. Furthermore, it does not provide any means for prolonging the fatigue life of existing one-piece fuel nozzle air swirler.
Therefore, manufacturing of new head components to avoid fatigue cracking due to thermal stresses, as well as reconditioning of operated components for extending the operating life thereof is highly desirable.
It is therefore an aim of the present invention to provide means for relieving thermal stress in a combustion chamber fuel nozzle of a gas turbine engine with minimum impact to the nozzle aerodynamics.
It is also an aim of the present invention to extend the life of a gas turbine fuel nozzle.
It is a further aim of the present invention to provide a method for improving the fatigue life of a thermally stressed portion of an aerodynamic surface of a gas turbine engine.
Therefore, in accordance with the present invention, there is provided a fuel nozzle for a combustor in a gas turbine engine. The fuel nozzle comprises a fuel nozzle body having a fuel inlet port at one end and a spray tip at the other end for atomizing the fuel. The spray tip includes a nozzle head defining a plurality of air passages for conveying hot pressurized air into the combustor. Each pair of adjacent air passages defines a web. The nozzle head has at least one stress-relief slit which extends through one of the air passages for reducing thermally induced stresses in the webs during operation. The stress-relief slit is sized to substantially prevent air leakage from the air passage.
In accordance with a further general aspect of the present invention, there is provided a method for reducing thermal stresses in a gas turbine engine fuel nozzle of the type having a nozzle head defining an array of air passages, the method comprising the steps of: selecting at least one of the air passages, and defining a stress-relief slit through each selected air passage.
In accordance with a still further general aspect of the present invention, there is provided a method for improving the fatigue life of a gas turbine engine part having an aerodynamic surface defining a fluid flow path, the method comprising the steps of: identifying a first location on said aerodynamic surface which is prone to cracking due to thermal stress, relieving stress from said first location by forming an appropriate number of stress-relief slits in said aerodynamic surface at a second location remote from said first location, said stress-relief slits being sized to substantially prevent fluid leakage from said fluid flow path through said stress-relief slits.